csky: issues with gcc 16.2.0 generated image, use gcc 15 for now

target/config/Config.in.compiler

@@ -25,6 +25,7 @@ default ADK_TOOLCHAIN_GCC_8 if ADK_TARGET_ARCH_PPC_WITH_SPE # last gcc version s
 default ADK_TOOLCHAIN_GCC_10 if ADK_TARGET_ARCH_CRIS # no longer supported
 default ADK_TOOLCHAIN_GCC_10 if ADK_TARGET_CPU_SH_J2 # j2.patch
 default ADK_TOOLCHAIN_GCC_14 if ADK_TARGET_ARCH_NIOS2 # no longer supported
+default ADK_TOOLCHAIN_GCC_15 if ADK_TARGET_ARCH_CSKY # system does no longer boot in qemu
 default ADK_TOOLCHAIN_GCC_16
 
 config ADK_TOOLCHAIN_GCC_GIT